Field method for detecting deposits containing uranium or thorium

ABSTRACT

Locations of buried deposits containing uranium are determined by detecting the presence of Pb214, Bi214, Po214, Pb210, Bi210 and Po210 in solutions obtained by chemically leaching these elements from rocks and soil. Alpha sensitive films are exposed to the alpha decay of Po214 and Po210. The films, when etched, will reveal the damage caused by the passage of the high energy alpha particles. Alpha damage as a function of sample size, volume of solution used, film area and exposure measures concentration of Po214 and Po210 in the sample. Anomalous concentrations suggest presence of buried deposits containing uranium. Similar anomalous concentrations of alpha damage from Bi212 and Po212 formed in films exposed to foils or planchets from leach solutions containing Pb212, Bi212 and Po212 suggest deposits containing thorium. Plotting normalized alpha damage (numbers of alpha damage per gram, per square mm., per hour) and sample locations suggest mineral deposit locations.

This is a continuation-in-part of application Ser. No. 879,578 filed Feb. 21, 1978, by Lorin R. Stieff.

BACKGROUND OF THE INVENTION

This invention relates generally to geological exploration techniques and more particularly has reference to geological testing of rock and soil samples to determine the location of buried deposits containing uranium or thorium.

DESCRIPTION OF THE PRIOR ART

Pertinent United States and foreign patents are found in Class 250, subclasses 253, 255, 364, Class 23, subclasses 230EP and 230R and Class 73 of the official Classifications of Patents in the United States Patent and Trademark Office.

Examples of pertinent patents are United States Patents U.S. Pat. Nos. 3,665,194, 3,825,751, 3,968,371, 3,988,587, 4,055,762, 4,064,436.

U.S. Pat. No. 3,665,194 shows a radon dosimetry system. A sheet of track registration material having the property of forming damage tracks along paths traversed by alpha particles is exposed in an area to be monitored. Alpha particle tracks are formed when the material becomes irradiated. The tracks are counted and serve as a measure of the amount of radon and uranium daughter products present.

U.S. Pat. No. 4,055,762 shows a radon daughter dosimeter. The device is designed to detect radon gas alpha daughters in ambient air.

U.S. Pat. No. 3,825,751 shows a method and apparatus for detecting radioactive substances by measuring emitted gamma radiation. The process includes aerial radioactivity surveying.

SUMMARY OF THE INVENTION

The present invention overcomes the problems which exist in the prior art devices.

The present invention is based on evidence that daughter products of uranium in the U238 decay chain migrate and fractionate in the process of forming low concentration halos surrounding the deposit. The present invention detects the presence of Pb214, Bi214, Pb210 and Bi210 precursors using films which record alpha track damage. The films may be loaded in the field with a Pb-Bi-Po solution obtained from soil or rock samples or exposed to a planchet on which the Po (and Pb plus Bi) in the solution has been deposited. The concentration of alpha damage as a function of sample size, volume of solution used, film area and exposure time is a measure of the presence of daughter products surrounding a buried deposit containing uranium or thorium or radium. Similarly, deposits containing thorium are detected by looking at solutions containing Pb212, Bi212 and Po212 as indicated by characteristic alpha particle decays of Bi212 and Pi212. A chart of decay schemes of uranium and thorium is shown in the drawing.

OBJECTS OF THE INVENTION

Objects of the invention are to provide an improved method for detecting deposits containing uranium or thorium, and to provide a radioactive mineral deposit detecting technique which detects the presence of daughter products below the surface of the ground.

Another object of the invention is to provide a method for detecting deposits of material having characteristic matter surrounding the deposit including collecting samples from beneath the surface of the ground in the vicinity of a deposit, studying the samples for characteristic matter, and calculating the location of the deposit based on the characteristic matter in the samples.

These and other and further objects and features of the invention are apparent in the disclosure which includes the above and below specifications and claims.

DETAILED DESCRIPTION OF THE INVENTION

Generally, the present invention is a method for exploring for mineral deposits, in particular, deposits containing daughter products in the uranium and thorium decay chain which migrate away from the ore deposit and are fractionated in the process of forming very low concentration halos surrounding the deposit.

In the case of U238, the daughter products Th234, U234, Th230, Ra226 and Pb210, because of their relatively long half lives, different geochemical behavior and, in the case of Rn222, its physical behavior as a noble gas, participate in migration, transport and fraction processes. The present invention determines the location of buried uranium deposits by detecting the presence of radon and indirectly the other daughter products above radon in the decay chain. More particularly, the present techniques detect the presence of Pb214 and Bi214, as well as Pb210 and Bi210 by alpha activity of Po214 and Po210, respectively.

Subsurface soil or rock samples are collected in the vicinity of a suspected U238 deposit. Since most radon and other daughter products migrating away from a U238 deposit do not reach the surface but rather decay in the ground, it is important that subsurface samples be collected. Pb-Bi-Po solutions are obtained from the samples using any standard geochemical prospecting techniques for Pb and Bi, for example, either a weak HNO3 or ammonium citrate leach of the sample followed by dithizone extraction and, if necessary, back extraction of the dithizone with 0.02N HNO3.

The Pb-Bi-Po solution may be loaded in the field on two planchets one of the planchets is then exposed to a film for a short period of time to record the decay of Po214 while the other planchet is exposed for a longer time to a film to record the decay of Po210. Alternatively, Pb, Bi or Po may be plated on the planchet and the films exposed to the planchets or plates. Po may be plated directly on the silver foils for exposure of the films, in one example. The alpha activity of the planchets may also be established using electronic instrumentation.

Alpha particles from polonium 214 and 210 in the solution impinge on the films and are recorded by the films. The films, after development, are examined microscopically for Po214 and Po210 alpha tracks. Po214 is used as the short term integrator of the movement of Rn222 and other daughter products through the soil. The Po210 decay is used as the long term integrator of Rn222 and other daughter products and is an indication of the presence of the immediate precursor pair Pb210-Bi210. As shown in the drawing, Po214 results from decay of elements having half lives of minutes; Po210 results from decay of elements having half lives of years.

Population of alpha damage as a function of sample size, volume of solution used, film area and exposure is a measure of the concentration of daughter products in the sample. An anomalous population of alpha tracks would suggest presence of halos possibly containing very small concentrations of uranium or thorium and their daughter products and may be evidence for the presence and location of deposits containing uranium or thorium. Plotting the sample locations and normalized alpha track populations provides a means of identifying areas in which deposits may be found.

When looking for deposits containing thorium, alpha tracks characteristic of Bi212 and Po212 can be used.

While the invention has been described with reference to a specific embodiment, the exact nature and scope of the invention is defined in the following claims. 

What is claimed is:
 1. A field method for detecting location of a deposit containing uranium and for distinguishing between uranium or thorium containing deposits having characteristic radioactive daughter products near the deposit comprisingcollecting subsurface samples from a vicinity of the deposit, studying the samples for characteristic radioactive daughter products which are a mixture of Pb214, Bi214, Po214, Pb210 and Po210 detected by the alpha decay of Po214 and Po210 respectively in samples collected in the vicinity of a deposit containing U238, and wherein chemically extracting Pb, Bi and Po in a solution from the samples, exposing first and second alpha sensitive films to Po from the solution, exposing the first film to record decay of Po214, exposing the second film to record decay of Po210, etching the films to reveal the high energy alpha particle damage, studying the first film for Po214 alpha damage, studying the second film for Po210 alpha damage, and detecting presence of the deposit containing uranium based on the alpha particle damage in the films caused by characteristic radioactive daughter product matter in the samples, plotting sample locations and alpha particle damage populations, and determining locations of deposits containing uranium from the plotting.
 2. The method of claim 1 whereinthe samples are soil or rock.
 3. The method of claim 1 whereinthe characteristic radioactive daughter product matter are distributed in differentiated zones surrounding the deposit, and the samples are collected from the zones.
 4. The method of claim 1 wherein the process of studying comprisesdetermining the population of alpha damage on the etched films as a function of sample size, volume of solution used, film area and exposure time.
 5. The method of claim 1 wherein the process of exposing the samples comprises exposing planchets on which alpha active Po from the solution has been deposited to the film.
 6. The method of claim 1 wherein the studying comprisesdetermining the concentration of alpha damage on the first film as a function of sample size, volume of solutions and film area, said determination being a direct short term integrator of movement of radon 222, and indirectly of the movement of radium 226 and thorium 230 as well as uranium 234 and 238, from the deposit, determining the concentration of alpha damage on the second film as a function of sample size, volume of solutions and film area, and exposure, said determination being the long term integrator of the movement of Rn222 from the deposit and an indicator of the presence of Pb210-Bi210. 